Reinforced ARP cylinder head studs BMW M20B25

The BMW engine concerned

The M20B25 is BMW's 2.5-litre (2,494 cm³) inline six-cylinder petrol engine from the M20 family, with a single overhead camshaft and twelve valves. Naturally aspirated, it powers the 325i and 325iX (E30 saloon, Touring and convertible), the 525i (E34) and the Z1.

As standard, the cylinder head is held by Torx torque-angle bolts, to be replaced whenever they are removed (part numbers 11 12 1 726 478 + 11 12 1 722 021, the second being a supplementary Torx bolt). They are tightened to 30 Nm, then 90° and another 90°; this angle tightening stretches these bolts permanently.

Reinforced ARP cylinder head studs

Every combustion cycle pushes the cylinder head upwards. The head fasteners resist it by keeping a constant clamping pressure on the gasket; if that pressure drops, the seal is no longer guaranteed.

The ARP 201-4305 kit contains 14 ARP2000 studs with 12-point nuts and washers. Its instruction sheet sets the fitting order: the studs are first screwed hand tight into the block, the head is then installed, and the nuts are torqued, which gives a higher preload while sparing the block threads. ARP2000 is an ARP proprietary alloy, heat treated to reach 220,000 psi tensile strength, with threads rolled after heat treatment. Standard engine fasteners mostly belong to property classes 10.9 and 12.9 (150,800 to 176,900 psi minimum): ARP2000 therefore offers 24 to 46 % more margin. These ARP2000 studs in M10 x 1.5 are calculated to handle up to 800 hp on this six-cylinder engine (calculated estimate).

Technical specifications

Tightening torque

No torque is applied to the studs as they go into the block: they are screwed in by hand. The instruction sheet leaves the option of thread locker to mount them permanently, provided tightening is done before the product hardens.

A stud whose thread opens into a water passage receives ARP thread sealer.

Follow the tightening sequence recommended by the manufacturer and bring to torque in three stages; the stated values assume ARP lubricant on the threads and under the nuts.

The washers also receive ARP lubricant, as stated in the instruction sheet.

The torque figures above come from ARP instruction sheet 201-4305, revision applicable on the day the page went online. The official instruction sheet prevails in case of difference and describes the complete procedure, tightening sequence included. Read the ARP 201-4305 instruction sheet

Technical guide

All torque values on this page are given with ARP assembly grease on the threads and under the nuts; another lubricant would make them wrong.

The ARP brand

ARP (Automotive Racing Products) was born in the early 1970s from the initiative of Gary Holzapfel, an engineer from the aerospace industry, determined to eliminate engine failures caused by weak fasteners in motorsport. The company designs and manufactures thousands of parts in California, in several material grades suited to each use.

Advantages over the original bolts

Criterion	Original head bolts	Reinforced ARP studs
Tightening method	Torque then angle (30 Nm + 90° + 90°)	Torque only, in 3 stages
Block threads	Stressed at every tightening	Preserved (stud fitted hand tight)

When studs become necessary

The original fasteners are sized for the engine as it leaves the factory. They need to be reinforced when:

• Forced induction is added or pushed beyond its original value
• The compression ratio rises as the build progresses
• The original gasket is replaced by a reinforced MLS gasket, which requires a high and uniform preload
• The engine has already suffered a lifted head or a gasket leak
• Competition use imposes severe heat cycles and regular head removals

Vehicle compatibility

OEM reference: 11 12 1 726 478 + 11 12 1 722 021

Why is it the most cost-effective solution?

The original head bolts, tightened to torque then angle, remain permanently stretched. Stronger, the ARP2000 fasteners withstand a much higher combustion pressure, forced induction included: they limit the risk of gasket leaks or breakage, and so avoid a rebuild far more expensive than the kit.

Consider a reinforced head gasket

Reinforced fasteners and the head gasket work together: the gasket must hold the same pressure as the fasteners. The matching gasket for this engine: